1. Field of Use:
This invention relates generally to warehouse cranes such as used to load storage stations arranged in horizontal rows and vertical columns on either side of a longitudinal path on which the crane travels. In particular, it relates to a crane which permits independently raising and lowering a pair of carriages, each of which is mounted on a separate mast of a double-masted warehouse crane.
2. Description of the Prior Art:
Warehouse cranes of both single- and double-masted construction are in wide use for loading and unloading materials from a rack comprising storage stations arranged in horizontal rows and vertical columns. These cranes typically shown in U.S. Pat. No. 3,830,379, issued Aug. 20, 1974, to Dechantsreiter, can travel longitudinally to place the loading mechanisms thereof adjacent any of the rack's vertical columns. Carriages to which the loading mechanisms are attached and which are vertically positionable on masts enable placement of the loading mechanism adjacent any of the rack's horizontal rows. Having thus been placed adjacent the desired storage station in a given vertical column and horizontal row, the loading or shuttle mechanism is moved transversely to the longitudinal path of the crane into the storage station, either to place a load into or remove a load from that station. Because the load on the shuttle mechanism may shift during this transverse movement, and because such shifting may jam the carriage when it is entering or leaving the storage station, preventing its vertical movement, a worker must reposition the load to free the carriage. With current single- or double-masted cranes, if the carriage is at a storage station in a horizontal row at a high elevation he has no alternative but to climb a stationary ladder on the crane to reach the shifted load, all of which is time consuming and laborious.
A carriage on current cranes is frequently limited in its movement by another carriage or carriages mounted thereon, whether the plurality of carriages is mounted on one or more masts. For example, U.S. Pat. No. 3,631,940, issued Jan. 4, 1972, to Richins, describes a double masted warehouse crane having both a load carriage and an operator's carriage. However, movement of the two carriages therein is interrelated with a lost motion switch, whereby the load carriage may move upwardly for a given distance independently of the operator's carriage but lifts the operator's carriage vertically upward therewith after that distance has been exceeded. This limitation can prevent access to the load carriage by the operator's carriage when the former is at certain heights. Additionally, each carriage described in Richin's is mounted to both of the masts.